1. Field of the Invention
The present invention relates to a high moment bilayer first pole piece layer of a write head with high magnetic stability for promoting read signal symmetry of a read head and, more particularly, wherein a magnetic moment of the first pole piece layer returns to the same orientation after being subjected to multiple instances of a write current field.
2. Description of the Related Art
The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm above the rotating disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads arc directly mounted on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent the ABS of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic bits to and reading magnetic bits from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a nonmagnetic gap layer at an air bearing surface (ABS) of the write head. The pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic field into the pole pieces that fringes across the gap between the pole pieces at the ABS. The fringe field or the lack thereof writes information in tracks on moving media, such as in circular tracks on a rotating disk.
It should be understood that the second pole piece is made of a magnetic material. A magnetic moment of the first pole piece layer should be oriented along an easy axis parallel to the ABS and perpendicular to the direction of movement of the track on the rotating disk when the first pole piece is in a quiescent state, namely without a write current field from the write coil. When the magnetic moment does not return to an orientation parallel to the ABS after being subjected to multiple instances of the write current field, the first pole piece is not stable and this will affect the operation of the read head, which will be discussed in more detail hereinafter. A factor bearing upon the magnetic stability of the first pole piece and the return of its magnetic moment to an orientation parallel to the ABS is the uniaxial anisotropy (H.sub.K) of the first pole piece. The uniaxial anisotropy (H.sub.K) is a measure of the amount of applied magnetic field required to rotate the magnetic moment of the first pole piece from the orientation parallel to the ABS to an orientation perpendicular to the ABS. If the uniaxial anisotropy is too low the magnetic moment may not always return parallel to the ABS, which will impact the operation of the read head. However, if the uniaxial anisotropy is too high, rotation of the magnetic moment will be stiff in response to the write current field which will degrade the flux carrying capability of the first pole piece. Accordingly, the uniaxial anisotropy should be not too low or too high.
A material typically used for the first pole piece is nickel iron (Ni.sub.80 Fe.sub.20) since nickel iron is a soft magnetic material. Nickel iron also has a high magnetic moment or magnetization for conducting flux to the ABS. The more flux conducted the greater the strength of the magnetic bits of information impressed into the rotating magnetic disk. Accordingly, when the write signal is increased the track width of the pole tip of the second pole piece can be reduced for increasing the storage capability of the disk drive. A material with a higher magnetic moment than nickel iron is iron nitride (FeN). Iron nitride will conduct more flux and thereby permit the track width of the pole tip of the second pole piece to be narrower. Unfortunately, while the iron nitride has a high magnetic moment, it does not have good magnetic stability. This means that the magnetic moment will not return to the parallel position to the ABS after being subjected to multiple instances of the write current field.
The read head includes a read sensor which is connected to first and second lead layers for conducting a sense current (I.sub.S) through the read sensor. The read sensor and the first and second lead layers are located between nonmagnetic electrically insulative first and second read gap layers and the first and second read gap layers are located between ferromagnetic first and second shield layers. In a merged magnetic head the second shield layer of the read head and the first pole piece layer of the write head are a common layer. In a piggyback type magnetic head the second shield layer and the first pole piece layer are separate layers and are insulated from one another by a nonmagnetic insulation layer. Magnetic instability of the first pole piece layer is more of a problem in the merged magnetic head since the first pole piece layer is closer to the read sensor and its magnetic instability more directly influences the magnetization of the read sensor.
The read sensor is typically an anisotropic magnetoresistive (AMR) read sensor or a giant magnetoresistive (GMR) read sensor which is also known as a spin valve sensor. Both sensors have a ferromagnetic layer with a magnetic moment that is oriented in a predetermined direction to the ABS in a quiescent state, namely when the read sensor is not reading a read signal. The AMR sensor employs a single ferromagnetic layer with its magnetic moment oriented typically 45.degree. to the ABS. When the AMR read sensor is subjected to positive and negative going read signals from the rotating magnetic disk, the magnetic moment of the AMR read sensor rotates upwardly and downwardly respectively. This causes a change in the magnetoresistance of the sensor which is a function of sin.sup.2 .theta., where .theta. is the angle between the direction of the magnetic moment of the ferromagnetic layer and the direction of the sense current. In the AMR sensor, when the angle .theta. between the orientation of the magnetic moment of the ferromagnetic layer is parallel to the sense current, the magnetoresistance is at a maximum and when the magnetic moment of the ferromagnetic layer is perpendicular to the sense current the magnetoresistance of the AMR sensor is at a minimum.
In the GMR sensor a nonmagnetic electrically conductive spacer layer is located between a ferromagnetic pinned layer and a ferromagnetic free layer. The pinned layer has its magnetic moment pinned perpendicular to the ABS by exchange coupling with an antiferromagnetic (AFM) layer. The magnetic moment of the free layer is oriented parallel to the ABS and its rotation upwardly or downwardly in response to positive or negative going signals from the rotating magnetic disk causes a change in the magnetoresistance of the GMR sensor. Magnetoresistance is a function of cos .theta. where .theta. is the angle between the orientation of the magnetic moment of the free layer and the orientation of the magnetic moment of the pinned layer. When these orientations are parallel with respect to one another the resistance is at a minimum and when these orientations are antiparallel the resistance is at a maximum.
When the magnetic moment of the first pole piece layer of the write head does not return to the same orientation parallel to the ABS after being subjected to a write current field this will influence the orientation of the magnetic moment of the ferromagnetic layer of the AMR sensor or the free layer of the GMR sensor from the 45.degree. angle and the zero degree angle to the ABS to some other angle. This degrades the performance of the read head by increasing read signal asymmetry. The read head has symmetry when it produces equal positive and negative read signals in response to equal positive and negative signals from the rotating magnetic disk. If the magnetostatic coupling between the first pole piece layer and the read sensor causes the orientation of the magnetic moment of the read sensor to be at some positive angle, the read sensor will read an increased positive read signal in response to the read signal from the magnetic disk and a lower negative read signal in response to the negative read signal from the rotating disk. This is known in the art as positive read signal asymmetry. The opposite situation would be when the first pole piece layer influences the magnetic moment of the read sensor to be at a negative angle with respect to the ABS. In this condition the read sensor would have negative read signal asymmetry. Without influences from the write head a bias point is centered on a transfer curve of the read sensor where the transfer curve is a plot of the change in resistance of the read sensor in response to applied read signals from the rotating magnetic disk. When the read sensor has positive or negative read signal asymmetry the bias point is moved from a zero or middle position on the transfer curve to a location upwardly or downwardly on the transfer curve. A change in the direction of the magnetic moment of the first pole piece layer affects the bias point of the read sensor.
Accordingly, there is a strong-felt need to provide a first pole piece layer for a write head which has a high magnetic moment and yet is magnetically stable so that it does not cause read signal asymmetry of the read head.